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Disclosure of relationship: The contributors of this report have disclosed that they have no financial interest, relationship, affiliation, or other association with any organization that might represent a conflict of interest. In addition, this report does not contain any discussion
of unlabeled use of commercial products or products for investigational use.

Abstract

Introduction: Detecting a disease outbreak is more difficult when the exposure occurs in a workplace
but only
the patient's home address is available for analysis. In these situations, application of the customary
spatial scan statistic designed by Martin Kulldorff does not account for possible
differences between home and work addresses, thereby reducing the power of detection.

Objectives: This study examined whether modifying Kulldorff's spatial scan statistic to take into account
the movement of persons between home and work can improve detection of disease outbreaks when exposure occurs in the workplace.

Methods: The study region was partitioned into
m cells Z(1),. . . ,Z(m). L(k,i) is the proportion of the population living in cell
Z(k) that works at cell Z(i). For each cell Z(i), i = 1,. . . ,m, consider the r nearest cells from Z(i),
r = 1,. . . ,R as the location of a possible outbreak that occurs during working
hours. For each i and each r, build the
m zones Y(1),. . .
,Y(m), adjoining successively the residential cells indicating where the workers from the
r nearest cells from Z(i) live, in decreasing order of proportion of workers within these cells. The factors L(k,i) are used to compute the observed cases in the residential zones attributable to the contamination from workers at the r nearest neighbors of cell
Z(i). This quantity, with the corresponding expected number of cases, is used to build the modified spatial
scan statistic, similar to the usual spatial scan statistic. The modified scan statistic is computed
m²R times, and the maximum value obtained indicates the most likely pair of outbreak focus and associated residential area found. A Monte Carlo procedure is used to compute the p-value of the most likely pair. The study region consisted of 158
ZIP codes located near Norfolk, Virginia. The following three typical simulated clusters, with their
corresponding ZIP codes, are representative of much more extensive simulations:
1) Cluster A: 23601, 23606, 23607, 23661, 23666, 23668, and 23669; 2) Cluster B: 23601, 23602, 23606, 23665, 23666, and 23693; and 3) Cluster C: 23666 and 23669.

Results: Power evaluations of 0.85 (A), 0.70 (B), and 0.53 (C) were obtained by using the modified scan
statistic compared with 0.68 (A), 0.52 (B), and 0.42 (C) obtained by using Kulldorff's spatial
scan statistic.

Conclusion: Using a modified scan statistic that takes into
account the movement of persons between home and work might be a useful
complementary tool for the early detection of outbreaks in the workplace.
Through simulations, a statistically significant increase in power was observed compared with the usual spatial scan statistic.

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